The present invention relates to graft polymeric membranes in which one or more trifluorovinyl aromatic monomers are radiation graft polymerized to a polymeric base film, and methods for making same wherein the grafted polymeric chains are modified to incorporate ion-exchange groups. The resultant membranes are useful in dialysis applications, and particularly in electrochemical applications, for example as membrane electrolytes in electrochemical fuel cells and electrolyzers.
The preparation of graft polymeric membranes by radiation grafting of a monomer to a polymeric base film has been demonstrated for various combinations of monomers and base films. The grafting of styrene to a polymeric base film, and subsequent sulfonation of the grafted polystyrene chains has been employd to prepare ion-exchange membranes.
U.S. Pat. No. 4,012,303, reports the radiation grafting of xcex1,xcex2,xcex2-trifluorostyrene (TFS) to polymeric base films using gamma ray co-irradiation, followed by the introduction of various ion-exchange substituents to the pendant aromatic rings of the grafted chains. With co-irradiation, since the TFS monomer is simultaneously irradiated, undesirable processes such as monomer dimerization and/or independent homopolymerization of the monomer may occur in competition with the desired graft polymerization reaction.
U.S. Pat. No. 4,012,303 also reports that the TFS monomer may be first sulfonated and then grafted to the base film. Thus, the introduction of ion-exchange groups into the membrane can be done as part of the grafting process, or in a second step.
More recently, the grafting of TFS to pre-irradiated polymeric base films, followed by the introduction of various substituents to the pendant aromatic rings of the grafted chain has been reported in U.S. Pat. No. 4,605,685. Solid or porous polymeric base films, such as for example polyethylene and polytetrafluoroethylene, are pre-irradiated and then contacted with TFS neat or in solution. Pre-irradiation is reportedly a more economic and efficient grafting technique, reportedly giving a percentage graft of 10-50% in reaction times of 1-50 hours. Aromatic sulfonation, haloalkylation, amination, hydroxylation, carboxylation, phosphonation and phosphorylation are among the reactions subsequently employd to introduce ion-exchange groups into the grafted polymeric chains. Levels of post-sulfonation from 40% to 100% are reported.
In either case the prior art TFS-based grafted membranes incorporate statistically a maximum of one functional group per monomer unit in the grafted chain. Further, they typically incorporate only one type of functional group as substituents on the pendant aromatic rings in the grafted chains.
In the present membranes, one or more types of substituted TFS monomers and/or substituted xcex1,xcex2,xcex2-trifluorovinylnaphthylene (TFN) monomers are grafted to polymeric base films, the substituents being selected to offer particular advantages, for example:
(a) Substituted TFS and/or TFN monomers that are activated have increased reactivity in the grafting reaction facilitating graft polymerization. By xe2x80x9cactivatedxe2x80x9d it is meant that either the percentage graft yield of the graft polymerization reaction is increased, or that the rate of the reaction is increased, in reactions employing the substituted monomers relative to reactions employing unsubstituted monomers.
(b) Substituted TFS and/or TFN monomers in which the substituents are activating with respect to the grafting reaction, but which can be converted so as to be de-activating with respect to subsequent reactions to introduce, for example, ion-exchange functionality into the grafted chains, and thereby permit the introduction of ion-exchange groups that are more stable under certain conditions.
(c) Substituted TFS and/or TFN monomers in which the substituents are activating with respect to the grafting reaction, but which can be converted so as to be de-activating after introduction of ion-exchange functionality into the grafted chains.
(d) Grafted chains comprising monomer units with more than one aromatic ring permit the introduction of more than one ion-exchange group per grafted monomer unit, enabling the achievement of higher ion-exchange capacities at lower percentage grafts than in prior art grafted polymeric membranes.
(e) Substituted TFS and/or TFN monomers in which the substituents are precursors to ion-exchange groups may be transformed to ion-exchange groups after the grafting reaction, and can facilitate the introduction of more than one type of ion-exchange group into the grafted chains, for example, so that both cation and anion-exchange groups may be incorporated in a membrane.
(f) Substituted TFS and/or TFN monomers in which the substituents contain functionality that can be further reacted to allow for the preparation of crosslinked graft polymeric membranes that may display, for example, greater dimensional stability under certain conditions than similar graft polymeric membranes that are not crosslinked.
A graft polymeric membrane is provided in which one or more types of trifluorovinyl aromatic monomers are graft polymerized to a polymeric base film. In one embodiment, the membrane comprises a polymeric base film to which has been graft polymerized a monomer (meaning at least one type of monomer) selected from the group consisting of monomers of the following formulae (I) and (II): 
where A1, A2, and B1, B2 are independently selected from the group consisting of hydrogen, lower alkyl, lower fluoroalkyl, cyclic alkyl, cyclic amine, cyclic ether, cyclic thioether, Ar (with the proviso that where one of A1 and A2 is hydrogen, Ar is other than Ph), CH(X)Ph, where X is selected from the group consisting of hydrogen, fluorine, lower alkyl, lower fluoroalkyl and Ph, PRRxe2x80x2 and P(OR) (ORxe2x80x2), where R and Rxe2x80x2 are independently selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and where R and Rxe2x80x2 can be the same or different); and, wherein A1, A2, B1, and B2 can be the same or different, provided that in the selected monomer at least one of the substituents A1, A2, B1, B2 is other than hydrogen. In other words there is at least one of the foregoing substituted monomers employd in the graft polymerization reaction. The selected substituted monomer(s) may have one or two non-hydrogen substituents.
Of the listed alkyl substituents, lower alkyl and cyclic alkyl are generally preferred, with methyl (Me) being most preferred. Thus, membranes where one or both substituents on the selected monomer of formula (I) or (II) are Me are particularly preferred, with para-Me being the most desirable substitution position in formula (I)). In these embodiments the base film preferably comprises poly(ethylene-co-tetrafluoroethylene).
In embodiments in which a polymeric base film has been graft polymerized with a monomer of formula (I) in which A1 is Ar and A2 is hydrogen, Ar is preferably a fused polycyclic aromatic with two fused rings, biphenyl, or a heteroaromatic group with at least one heteroatom that is preferably nitrogen, oxygen or sulfur. If the heteroaromatic group contains more than one heteroatom, the heteroatoms may be the same or different. If one of the heteroatoms is nitrogen it may be advantageously N-alkylated or N-benzylated for certain membrane applications. Monocyclic heteroaromatics are generally preferred over polycyclic heteroaromatics.
The above graft polymeric membrane may comprise a single monomer, whereby the grafted chains are homopolymeric, or may comprise more than one monomer such that the grafted chains are copolymeric. For example, the graft polymeric membrane may comprise more than one monomer of formula (I) having different A1 and/or A2 substituents, more than one monomer of formula (II) having different B1 and/or B2 substituents, more than one monomer of either formula (I) or formula (II) having the same substituents located at different positions, or monomers of both formula (I) and (II), such that the grafted chains are copolymeric.
In another embodiment of the present graft polymeric membrane, the membrane comprises a polymeric base film to which has been graft polymerized, with the foregoing monomers, a monomer of the following formula (III): 
where D is selected from the group consisting of hydrogen, fluorine, CF3, CF2H, CFxe2x95x90CF2, SO2F and SO3xe2x88x92 M+.
Embodiments of the present graft polymeric membrane comprise a polymeric base film with grafted chains comprising monomer units selected from the group consisting of monomer units of the following formulae (IV) and (V), wherein at least a portion of the monomer units further optionally comprise at least one ion-exchange substituent, in which case the membrane is an ion-exchange membrane: 
where, as before, A1, A2, and B1, B2 are independently selected from the group consisting of hydrogen, lower alkyl, lower fluoroalkyl, cyclic alkyl, cyclic amine, cyclic ether, cyclic thioether, Ar (with the proviso that where one of A1 and A2 is hydrogen, Ar is other than Ph), CH(X)Ph, where X is selected from the group consisting of hydrogen, fluorine, lower alkyl, lower fluoroalkyl and Ph, PRRxe2x80x2 and P(OR) (ORxe2x80x2), where R and Rxe2x80x2 are independently selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and where R and Rxe2x80x2 can be the same or different); and wherein A1, A2, B1, and B2 can be the same or different, provided that at least one of the substituents A1, A2 is other than hydrogen. The foregoing membranes may be formed by grafting monomers to a polymeric base film, or by grafting to some other form of polymeric substrate and then forming the grafted material into a membrane. In some embodiments of the ion-exchange membranes, statistically at least 50% of the monomer units in the grafted chains have at least one ion-exchange substituent per monomer unit. In other embodiments at least a portion of the monomer units comprise more than one ion-exchange substituent, and/or portion of the grafted chains may comprise at least two different types of ion-exchange groups, which may even include both anion and cation exchange groups. The ion-exchange substituent most typically incorporated is a sulfonate or sulfonic acid group.
In preferred embodiments one or both substituents of the monomer units of formulae (IV) or (V) are CH(X)Ph, where X is selected from the smaller group consisting of hydrogen, fluorine, Me and Ph, or Me, with para-Me being the most desirable substitution position for the Me group in units of formula (IV). In these embodiments, again, the base film preferably comprises poly(ethylene-co-tetrafluoroethylene).
The grafted chains of ion-exchange membrane may further comprise additional monomer units, such as for example, units of formula (VI): 
where D is selected from the group consisting of hydrogen, fluorine, CF3, CF2H, CFxe2x95x90CF2, SO2F and SO3xe2x88x92 M+.
The ion-exchange membrane may be substantially gas impermeable. Such impermeable ion-exchange membranes may be incorporated into an electrode apparatus such as, for example, a membrane electrode assembly. Electrochemical fuel cells that comprise such ion-exchange membranes are also provided. For fuel cell applications, the polymeric base film of the ion-exchange membrane is preferably less than 100 xcexcm thick.
In the present graft polymeric membranes or ion-exchange membranes, at least a portion of the grafted chains may be crosslinked.
Other membranes may be prepared from those membranes described above by subjecting them to a reaction process selected from the group consisting of, for example, halomethylation, sulfonation, phosphonation, amination, carboxylation, hydroxylation and nitration. Membranes so prepared may be useful ion-exchange membranes or precursors to ion-exchange membranes. Methods of preparing the present membranes and ion-exchange membranes are also provided.
Ion-exchange membranes may be prepared by a method which comprises graft polymerizing to a polymeric base film a monomer selected from the group consisting of monomers of formulae (I) and (II) described above, wherein in the selected monomer(s) at least one of the substituents A1, A2, and B1, B2 is a non-hydrogen substituent which activates the monomer with respect to graft polymerization (relative to the corresponding unsubstituted monomer). The method further comprises introducing a sulfonate group (or other ion-exchange group) into at least a portion of the graft polymerized monomer units and converting at least a portion of the non-hydrogen substituents to substituents which are deactivating with respect to desulfonation (relative to the unsubstituted monomer unit). The conversion of the non-hydrogen substituent to a deactivating group may be performed before or after introduction of the sulfonate group into the grafted units.
Some of the membranes described above may be prepared by a method comprising graft polymerizing to a polymeric base film a substituted monomer selected from the group consisting of monomers of formulae (I) and (II) described above, wherein A1, A2, and B1, B2 are as described above.
In preferred embodiments of this method, A1 and B1 are independently selected from the group consisting of:
Ar, where Ar is selected from the group consisting of monocyclic heteroaromatics, fused polycyclic heteroaromatics, and heteroaromatic ring assemblies having at least one nitrogen atom);
cyclic amine; and
phosphines of the formula PRRxe2x80x2 and phosphites of formula P(OR) (ORxe2x80x2), where R and Rxe2x80x2 are independently selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and where R and Rxe2x80x2 can be the same or different); and
A2 and B2 are hydrogen.
The method further comprises alkylating or benzylating at least a portion of any of the nitrogen atoms of the Ar group, the nitrogen atoms of the cyclic amine, or the phosphorus atoms of the phosphine or phosphite.
In other embodiments where A1 and B1 are independently selected from the group consisting of phosphines of the formula PRRxe2x80x2 and phosphites of formula P(OR) (ORxe2x80x2), where R and Rxe2x80x2 are independently selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and where R and Rxe2x80x2 can be the same or different), and A2 and B2 are hydrogen, the method may further comprise the sequential steps of introducing a nitro group into at least a portion of the monomer units of the membrane and converting at least a portion of those nitro groups to quaternary ammonium groups. This method optionally further comprises subsequently converting said phosphine or phosphite to an ion-exchange substituent.
In still another embodiment, the present method comprises graft polymerizing to a polymeric base film a monomer selected from the group consisting of monomers of the formulae (I) and (II) described above, but where A1 and B1 are independently selected from the group consisting of PRRxe2x80x2, P(OR) (ORxe2x80x2), and SR, where R and Rxe2x80x2 are independently selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and where R and Rxe2x80x2 can be the same or different), and A2 and B2 are the same as A1 and B1 respectively or hydrogen. The method comprises the steps of graft polymerizing the monomers to a polymeric base film, and oxidizing at least a portion of the PRRxe2x80x2, P(OR) (ORxe2x80x2), or SR groups to produce phosphine oxides, phosphones, phosphonates, sulfoxides, or sulfones. The method may further comprise introducing ion-exchange substituents into at least a portion of said monomer units, before or after the oxidation step. Where A1 and B1 are independently selected from the group SR, where R is selected from the group consisting of lower alkyl, cyclic alkyl and Ph, and A2 and B2 are the same as A1 and B1 respectively or hydrogen, the method optionally further comprises converting at least a portion of the SR groups to sulfonate or sulfonic acid groups.
In the above-described embodiments the substrate for the graft polymerization is preferably a polymeric base film. However, the polymeric substrate may be in other forms such as, for example, a powder or in solution, or the substrate may be an oligomer in any form. Where the substrate is not in the form of a film an additional step will be required to form the grafted material into a membrane. Where the substrate is in solution an additional solvent removal step will be required.